Hitherto, there have been developed a variety of nitrogen-oxide-removing catalysts which selectively catalyze reduction of nitrogen oxide with NH3. For example, there have been proposed, as such nitrogen-oxide-removing catalysts, a catalyst containing β-zeolite in which metal element ions have been substituted by iron ions and lanthanum ions (see Patent Document 1), and a catalyst formed of zeolite in which metal elements have been substituted by Fe ions and rare earth metal element ions (see Patent Document 2).
However, these conventional catalysts lack sufficient activity for selective reduction of nitrogen oxide and sufficient catalyst durability.    [Patent Document 1] Japanese Patent Application Laid-Open (kokai) No. 2005-177570    [Patent Document 2] Japanese Patent Application Laid-Open (kokai) No. 2006-305423